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Article

Comparative Performance Evaluation of Retrofit Alternatives for Upgrading Simply Supported Bridges Using 3D Fiber-Based Analysis

by
Homam Ghazal
1 and
Aman Mwafy
1,2,3,*
1
Department of Civil and Environmental Engineering, United Arab Emirates University, Al Ain 15551, United Arab Emirates
2
School of Engineering, Faculty of Applied Science, University of British Columbia, Okanagan, Kelowna, BC V1V 1V7, Canada
3
Department of Structural Engineering, Faculty of Engineering, Zagazig University, Zagazig 44519, Egypt
*
Author to whom correspondence should be addressed.
Buildings 2023, 13(5), 1161; https://doi.org/10.3390/buildings13051161
Submission received: 5 April 2023 / Revised: 21 April 2023 / Accepted: 25 April 2023 / Published: 27 April 2023
(This article belongs to the Special Issue Seismic Assessment and Rehabilitation of Reinforced Concrete (RC) Structures)
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Abstract

:
This study aims to select an effective mitigation approach from different alternatives to upgrade substandard RC bridges to meet the seismic performance objectives of current design standards. The performance assessment results for an existing benchmark bridge confirmed that the bent curvature ductility and bearing displacement control the seismic response. Thus, five contemporary retrofit solutions were investigated, including adding different supplementary lateral force-resisting systems (SLFRSs), replacing old bearings with those equipped with shape memory alloy (SMA), and combinations of these retrofit options. Fourteen earthquake records representing long- and short-period seismic events and the seismo-tectonic characteristics of a moderate seismic region were progressively scaled and applied separately in the two orthogonal directions of detailed simulation models representing the retrofitted benchmark bridge. This study provided insights into the impact of combining contemporary seismic risk mitigation techniques on improving the seismic performance of substandard bridges and presented a range of fragility functions for delaying structural damage and minimizing disruption of existing bridges to avoid traffic interruption. The dynamic response simulation results in the longitudinal direction (LD) confirmed that utilizing SMA bearings reduces curvature ductility and bearing displacement demands. Although the probabilistic assessment study in the transverse direction (TD) indicated that SMA bearings adequately reduce displacement demands, the bridge should be equipped with SLFRSs to overcome the bents’ high curvature ductility demands. Therefore, the most effective retrofit technique in TD is achieved using both SMA bearings and steel bracings.

1. Introduction

Previous vulnerability assessment studies and observations from strong earthquakes in different parts of the world confirmed that bridges designed following standards not conforming to modern design provisions might be severely damaged or experience collapse [1,2,3]. If it is unserviceable after earthquakes, the consequences of a bridge shutdown include severe traffic congestion around the closed bridge and considerable economic losses. Earthquake risk management systems help researchers and practicing engineers estimate and reduce the predicted losses in the bridge inventory at an urban scale under anticipated seismic events (e.g., [4,5]). Fragility functions that relate the ground motion intensity to the probability of exceeding limit states are essential to estimate and mitigate the physical earthquake losses, which involve direct (structural damage cost) and indirect (cost of interruptions to business) losses. Potential damage to existing substandard bridges can be mitigated using contemporary retrofit approaches. While it is common practice that the most deficient bridges, such as pre-seismic code structures, are retrofitted first, then the less deficient ones, such as recently constructed bridges, it is not recommended to retrofit standard bridges with costly techniques that have a small number of service life years [2,5,6].
Selecting an effective retrofit technique to mitigate seismic risk and guarantee continuous operation after an earthquake requires considering several parameters, including strength, stiffness, and ductility. Seismic mitigation techniques for improving the performance of existing bridges can be divided into two main categories: (i) those directed to reducing seismic demands and (ii) approaches for enhancing seismic performance, as shown in Figure 1. Either method can be applied to the whole structure or a specific member. For instance, reducing the structure’s mass reduces the inertia forces and seismic demands. Reducing earthquake effects can be completed using dampers. Moreover, improving the structural members’ strength, ductility, or stiffness enables the bridge to withstand higher seismic loads. As shown in Figure 1, several retrofit approaches were recommended in the literature to improve the seismic performance of existing bridges. Conventional seismic retrofit approaches can upgrade RC structures. However, they have several drawbacks as they require special detailing, surface preparation, and formwork and may be adversely affected by increased temperatures [7,8]. It should be noted that the retrofit techniques related to reducing seismic demands can be directed to the structure or the foundation. For instance, geotechnical seismic isolation (GSI) is an emerging technique to reduce the seismic energy transmitted to the structure by improving the soil dynamic properties using, for instance, injectable material to reduce earthquake accelerations at the foundation level [9]. Previous studies confirmed the effectiveness of the GSI technique for mitigating the seismic risk of different bridges [10].
Using isolation bearings as a seismic retrofit measure replaces vulnerable bridge bearings and protects other structural components from damage. These bearings reduce the accelerations in the superstructure and consequently lower inertia forces. As a result, the relative displacement between the superstructure and substructure tends to increase yet is kept at acceptable levels by the energy dissipation provided by the bearing. Shape memory alloy (SMA) bearings are a contemporary isolator with recentering and adequate energy dissipation capacity. Previous studies investigating the impact of SMA bearings on upgrading bridge performance highlighted the higher energy dissipation capacity of smart bearings equipped with SMA wires compared to conventional isolators [11,12]. Limiting the residual deformation and shear strain demands was the main advantage of using lead rubber bearings (LRBs) with SMA wires [12,13].
Moreover, braced frames were extensively used in retrofit steel and RC buildings and bridges [14,15]. However, conventional steel bracings have some disadvantages, mainly their inability to resist high compression forces. Hence, a special steel brace that can effectively resist axial compression through controlling buckling is selected in this study as a retrofit approach when combined with SMA bearings. Buckling-restrained braces (BRB) can enhance ductility and post-yielding performance. Moreover, the light weight of steel braces is likely to have a marginal impact on a bridge’s seismic weight and inertia forces. Therefore, steel braces can be designed to yield earlier than main structural elements. Thus, they work like structural fuses that protect the bridge and may be replaced after the earthquake. Previous dynamic response simulation and experimental studies covering bridges retrofitted using steel braces that can restrain buckling highlighted their effectiveness in dissipating seismic energy and controlling damage [15,16]. Buckling-restrained bracings can be improved further by accounting for self-centering (SC) capabilities to overcome the residual displacement under severe earthquakes [17,18]. Different SC-BRBs were proposed in previous studies, which confirmed their flag-shaped hysteretic response with limited residual displacements [18,19]. Compared to conventional BRBs, SC bracing systems may adversely affect a structure’s acceleration response, which is a possible impact on the seismic response of bridges that warrants additional investigation. Therefore, this study investigates SC-BRB as a seismic risk mitigation alternative, particularly when combined with SMA bearings.
While previous seismic performance assessment studies focused on upgrading bridges, especially in high seismicity regions, the impact of combining different seismic risk mitigation alternatives for improving the performance of bridges in the United Arab Emirates (UAE) has not been investigated in depth. The limited studies in the selected case study area, a medium seismicity region subjected to multiple earthquake scenarios, mainly focused on a retrofit technique to overcome specific bridge deficiencies. For instance, a recent study on the retrofit of an old bridge concluded that while steel bracings effectively reduce ductility demands and the associated damage probability in one bridge direction, they marginally reduce bearing displacement demands [20]. The latter study suggested investigating additional retrofit techniques to mitigate bearing displacement demands in the two orthogonal directions of bridges. This brief literature review highlights the pressing need for further research to select a contemporary and most effective retrofit technique for existing bridges, particularly in the study region. This research gap is critical when investigating combinations of different retrofit systems for improving the seismic performance of substandard bridges in the longitudinal and transverse directions (LD and TD, respectively).
The present study thus is directed at the comparative performance assessment of a reference structure characterizing simply supported, pre-seismic code bridges when retrofitted with various contemporary retrofit alternatives along with their combinations. The main objective is to arrive at fragility functions for a representative archetype bridge retrofitted with different mitigation techniques and their combinations prepared for a regional risk mitigation system. The as-built reference bridge is compared in the present study with the following five retrofitted alternatives: (i) shape memory alloy (SMA) energy-dissipating bearings, (ii) conventional BRBs, (iii) self-centered BRBs, (iv) combined SMA bearings and BRBs, and (v) combined SMA bearings and SC-BRBs. Three-dimensional (3D) numerical models for a selected existing bridge and the retrofitted alternatives were developed and utilized to compare their seismic response under a diverse range of earthquake input ground motions reflecting the study region’s seismicity. Several fragility functions were developed and used to select the most effective retrofit technique or combination for the existing multi-span bridges in this region.

2. Description of Architype Bridge and Numerical Modeling with Retrofit Alternatives

2.1. Case Study Area and Reference Structure

Recent studies classified old structures designed and constructed in the selected construction site of the reference bridge in the early 2000s and before pre-code structures, since design codes previously considered the UAE a non-seismic zone [14,21]. However, recent seismic hazard studies considered the UAE a low-to-medium seismicity region vulnerable to short-period (near-source) and long-period (far-field) seismic events [22,23,24]. Therefore, pre-seismic code bridges in the case study region might be vulnerable to earthquake losses due to inadequate seismic provisions during the design and construction, particularly under different seismic scenarios, if not mitigated using suitable retrofit techniques. As a result, the need for a fragility assessment of pre-code bridges in the region is emphasized since they were constructed before the adoption of current seismic standards. A bridge database composed for the study area indicated that most of the bridges were multi-span span simply supported (MSSS) bridges. Therefore, the reference structure in the present study was chosen as a sample representing the typical bridge stock in the case study area. The selected bridge’s superstructure comprises five spans, each having an RC deck slab supported on four RC girders extending 14,000 mm, as depicted in Figure 2a. Rubber elastomeric bearings support the bridge superstructure. Expansion gaps of 40 mm separate adjacent superstructure spans. Similar expansion gaps are also present between the abutments and the superstructure. Rigid stoppers are used with a gap width of 20 mm to control excessive movement in the TD, as shown in Figure 2b. The bridge substructure consists of four two-column bents. The bent columns have a 1000 mm circular cross-section reinforced with 20#18 mm rebars and support an RC cap beam, as shown in Figure 2.

2.2. Description of the Numerical Modeling Approach

As this study focuses on the inelastic seismic response of simply supported bridges retrofitted with SLFRSs, bearings, and their combinations, detailed 3D numerical models are assembled for the retrofitted archetype bridge using an experimentally verified inelastic analysis platform [25]. The present study’s modeling approaches and analysis platform effectively predicted the inelastic dynamic response of complex structures and were verified against large-scale shake table tests [26,27]. Moreover, the utilized modeling approach was compared with quasi-static testing conducted for a bridge bent before and after its retrofit with different SLFRSs [28]. Different types of SLFRSs were used to retrofit the bridge bent [28]. The assembled 3D models for a two-column bent with retrofitting techniques were assessed under the effect of cyclic loadings used in the latter experimental study. Comparisons of the dynamic response simulation results with the cyclic loading experiments conducted in previous studies verified the idealization approach implemented in the current study [28,29]. The verification process ensures the accuracy of the adopted approach for idealizing various components of the existing and retrofitted benchmark bridge with SLFRSs.
As previously emphasized, SC-BRBs are improved SLFRSs compared to traditional BRBs in controlling residual deformations. In addition to verifying the modeling approach conducted in the present study for existing and retrofitted bridge bents with traditional LFRSs, previous studies involving quasi-static testing are utilized to verify the SC-BRBs idealization technique, as shown in Figure 3a [30]. Moreover, the retrofit technique proposed to reduce the displacement demands of bridge bearings in this study is the SMA wire-based laminated LRBs. The SMA wires dissipate energy and provide the bearing with recentering capabilities [31]. The behavior of these bearings depends on the SMA wires and the LRBs used. Since LRBs may undergo large deformations, they are used with SMAs to control the residual displacements and limit the force transmitted to the superstructure [32]. Therefore, the SMA-LRBs are chosen in this study to retrofit the selected benchmark bridge. The SMA-LRBs are idealized using a self-centering link element to arrive at the recentering and the flag-shaped behavior of the considered bearings, as shown in Figure 3b [32]. For the sake of brevity, the results of the comprehensive verifications conducted in the present study on the modeling approach for the benchmark bridge and its retrofit alternatives are presented by Ghazal and Mwafy [20].
Several displacement-based frame and link elements are utilized to develop the 3D model for the archetype bridge with different retrofit options. Different structural members of the substructure and superstructure are idealized in the 3D simulation model to evaluate the archetype bridge’s dynamic response, as depicted in Figure 2a. Since pounding between different bridge segments is expected during the dynamic response simulation under the effect of different input ground motions, the hysteretic response of bridge gaps should be effectively modeled. The 3D numerical models for the MSSS archetype bridge with the seismic risk mitigation alternatives consider the interaction between the structure’s segments. Considering the bridge deck, girders, and cross-beams in the 3D numerical model using inelastic, displacement-based frame elements provides further refinements to the developed 3D model and enhances its prediction for the dynamic response of the archetype bridge [25]. The main girders of the benchmark bridge are modeled using T-beams, with the flange representing the deck slab and the web representing the girders.
The force–deformation cyclic response of the expansion gaps in the two horizontal directions is obtained considering the gap width in LD (−40 mm, i.e., expansion gap closing) and TD (±20 mm, i.e., stopper gap width, Figure 2c). Moreover, the elastomeric bridge bearings have a length of 400 mm and 250 mm in the LD and TD, respectively. The force–deformation cyclic response of the elastomeric bridge bearings accounts for a maximum displacement of 50% of the bearing dimension [6,33]. The abutments modeling was undertaken using a simple hyperbolic force–deformation response according to the recommendations of previous studies for a comparable backfill of the abutments [34]. The transverse stiffness of the bridge abutments was calculated as 50% of the nearby bridge bent lateral stiffness, as shown in Figure 2a [35]. This stiffness is calculated using a linearized capacity envelope obtained from incremental static inelastic analysis for the bridge bent. The bent lateral capacity is used to model the bridge abutments in the TD.

3. Elements of the Comparative Performance Evaluation

This comparative performance assessment study considered fourteen long- and short-period input ground motions, characterized by their peak ground acceleration (PGA) to peak ground velocity (PGV) ratios (a/v), as shown in Table 1. Previous seismic performance assessment studies adopted the selected input ground motions to represent the case study region [23,36,37,38]. Table 1 summarizes the characteristics of the input ground motions used to assess the retrofitted archetype bridge.
Several previous studies investigated the significance of spatially varied input ground motions on the seismic response of extended bridges [2,39,40]. Simplifying such complex effects is challenging due to the random nature of asynchronous ground motions and the unpredictable effect on various bridge members [39]. Local site effects of overlying soil layers, which represents one of the effects of spatially varied earthquake ground motions, are accounted for in the present study by selecting input ground motions recorded at sites comparable to the soil class in the case study area. Hence, the selected input ground motions represent propagated bedrock earthquake records to the bridge foundation level. Previous studies concluded that other effects of asynchronous ground motions, including ground motion incoherency and wave passage effects, might amplify or suppress seismic demands of different bridge elements and should be considered for extended irregular structures constructed on thick and diverse layers of soil deposits. The bridge database collected from the study area indicated that most of the bridges have a limited number of short spans and are mainly located on granular soil deposits. Hence, the effects of ground motion incoherency and wave passage that result in different earthquake accelerations at different bridge supports were not accounted for in the present study, which focuses on selecting an effective retrofit approach from different alternatives for MSSS bridges using synchronous fragility analysis. Furthermore, previous studies also emphasized the importance of considering 3D numerical models that include the soil–foundation–structure interaction in seismic vulnerability assessments of complex bridges [1,41]. Since the bridges in the study area are constructed on competent soil deposits and generally supported with similar foundation systems comprising thick raft foundations, a simplification is adopted by modeling the granular soil at abutments using elastic springs, as shown in Figure 2a.
Around 600 time-history analyses were conducted in this study to understand the inelastic dynamic behavior of the retrofitted bridge using different techniques under the effect of the selected earthquake records. The input ground motions representing the long-period seismic scenario are scaled to increasing intensity levels, i.e., 0.16 g or 1 × Design to 6.0 × Design, with a 0.16 g increment. The short-period records are also scaled to different increasing intensity levels, i.e., 2.5 × Design to 15 × Design, with a 2.5 × Design increment. The selected scaling approach ensures that the adopted performance indicators for the comparative performance assessment of the archetype bridge are satisfied.
The relation between the performance limit state exceedance probabilities (LSEPs) and ground motion intensity (GMI) can be depicted using fragility functions, which account for the capacity and demand uncertainties. The fragility functions are typically integrated with seismic hazard and inventory databases to predict seismic losses and develop risk reduction strategies with the retrofit of existing structures. The fragility functions can be derived using various seismic analysis procedures involving simple or complex approaches such as incremental inelastic or dynamic analyses. Previous research studies adopted the incremental dynamic analysis (IDA) approach for developing fragility curves [14,26,37,42]. The IDA comprehensive approach was used in the present study as it considers or reduces uncertainties such as those related to input ground motions and numerical modeling. Since the inelastic multi-degree-of-freedom simulation is the most suitable alternative for predicting the inelastic dynamic response of complex structures, it is used in the current study to derive the archetype bridge’s fragility relations with different retrofit alternatives. Moreover, this study selected PGA as the GMI measure for developing fragility relations from different alternatives. This GMI measure complies with the recommendations of several previous studies [14,26,43].
Furthermore, seismic performance limit states were used in the present study based on the recommendations of previous studies related to bridges [20,44,45]. It was shown in a previous study that the damage of the selected multi-span bridge is associated with bent curvature ductility (CD) and bearing displacement (BD) demands. Therefore, for CD, the seismic performance indicators for the slight, moderate, extensive, and complete limit states (SL, MO, EX, and CO, respectively) are 1.3, 2.1, 3.5, and 5.2, respectively [44]. For BD, the seismic performance indicators for the same four limit states are 4, 10, 20, and 28 cm, respectively [45]. These limit states are based on the archetype bridge’s expansion gap, bearing, and unseating widths.

4. Probabilistic Performance Evaluation of Retrofit Techniques in the Longitudinal Direction

The CD and BD are initially monitored when the archetype bridge is assessed in two orthogonal directions. For the LD under the effect of earthquake records representing the far-field seismic scenario, the bridge retrofitted with SMA bearings has slightly higher CD demands, particularly at high input ground motion intensities, ranging from 3 × Design to 6 × Design (i.e., from 0.48 g to 0.96 g, respectively). In contrast, for the short-period seismic scenario, the CD demands of the retrofitted bridge with SMA bearings are lower than the as-built structure at all ground motion intensities, as shown in Figure 4. As for the BDs, it is seen in Figure 5 that the demands of the SMA-retrofitted bridge are lower than the as-built counterpart under both far-field and short-period records. Furthermore, it is shown from the sample results presented in Figure 4 and Figure 5 that the SMA wire-based laminated rubber bearing is not expected to mitigate the probability of damage in bridge bents since CD demands are related to the strength capacity and longitudinal reinforcement strain in columns. On the other hand, in the case of BDs, the SMA bearings can reduce displacement demands due to their self-centering ability.
As previously emphasized, inelastic multi-degree-of-freedom numerical models for the benchmark bridge with different retrofit alternatives are used to derive several fragility functions. Several IDAs were conducted by scaling and applying each of the selected fourteen input ground motions to the 3D models. The scaling factors were chosen to help detect the performance indicators for the comparative performance evaluation of the archetype bridge. In the LD of the structure, it is seen in Figure 6 and Figure 7 that under the long-period earthquake records, the CD and BD damage probabilities are noticeably reduced across all four performance limit states when the existing bridge bearings are replaced with SMA bearings. Under the short-period earthquake records, the LSEPs related to CD are slightly reduced, while the BD demands are significantly reduced by retrofitting the benchmark bridge using SMA bearings, as shown in Figure 8 and Figure 9. The results reflect the higher effectiveness of the SMA retrofit approach in reducing BDs than CD demands, which are more influenced by the retrofit approaches directed to bridge bents.

5. Probabilistic Performance Evaluation of Retrofit Techniques in the Transverse Direction

The most effective retrofit technique varies for different seismic scenarios and performance limit states in the TD. It is seen from the sample IDA results presented in Figure 10 that for long-period earthquake records, the most effective retrofit alternative resulting in the lowest CD is achieved by combining SMA bearings with SLFRSs (i.e., SMA-BRB), while the least effective retrofit approach is SMA bearings. For the short-period earthquake records, the most effective alternative in terms of CD demands is when retrofitting the bridge with SLFRSs (i.e., BRBs), while the highest CD demands are found with the SMA bearings (Figure 10). As shown in Figure 11, the lowest BD demands under the long-period input ground motions are observed using the SMA bearing retrofit technique, while the highest BDs are seen when using the SLFRSs retrofit approach (i.e., SC-BRB). Furthermore, the SLFRSs retrofit technique with BRBs provides the least BDs across all retrofit techniques, while the SMA bearing retrofit technique provides the highest BDs (Figure 11).
As previously highlighted, IDAs are undertaken in the TD using multi-degree-of-freedom numerical models for the benchmark bridge with different retrofit alternatives for deriving the related fragility functions. The dynamic response simulation results under long- and short-period seismic events reflect a noticeable reduction in the LSEPs and substructure CD demands when the archetype structure is equipped with the SLFRSs (i.e., BRBs), as shown in Figure 12, Figure 13, Figure 14 and Figure 15. Concerning BDs, the SLFRSs retrofit approaches with both conventional and SC BRBs negligibly affect the probability of damage due to the marginal impact of retrofitting bridge substructure with steel braces on the BD demands under both long- and short-period earthquake scenarios, especially for the SL and MO seismic response indicators. For the other performance criteria (i.e., EX and CO), the SLFRSs marginally increase the bridge BD demands (Figure 13 and Figure 15).
As a result of the abovementioned observations, the SMA bearings and their combination with SLFRSs were investigated to determine the most effective retrofit strategy for the benchmark MSSS bridge. The results indicate that retrofitting the bridge with a retrofit technique combination (RTC) using both SMA bearings and SLFRSs (i.e., SMA-BRB) significantly reduces the CD LSEPs under the long-period scenario (Figure 12). On the other hand, the most effective alternative to reduce the BD LSEPs is the SMA-bearing approach (Figure 13). Under the effect of the short-period seismic scenario, the SLFRS retrofit technique with BRBs provides the least CD LSEPs (Figure 14). However, the SMA bearing retrofit technique effectively reduces the BD LSEPs for the short-period seismic scenario (Figure 15).
The presented fragility curves in the TD indicated that if the BD demands are the most critical issue for the benchmark bridge, SMA bearings would be the most effective retrofit technique under both considered seismic scenarios. On the other hand, to overcome the high CD demands, the bridge should be retrofitted with SLFRSs. Using SMA bearings and SLFRSs (i.e., SMA-BRBs) further improves the performance under long-period records. It is important to note that the seismic response of the existing bridge in the TD was controlled by CD demands [20]. Hence, the recommended retrofit technique for the archetype bridge in TD combines SMA bearings and SLFRSs (i.e., SMA-BRB), which is effective under both long- and short-period earthquake scenarios, in particular, the former scenario that has a significant effect on different types of structures in the UAE [14,26,36,37].

6. Discussion of the Performance Evaluation Results for Different Retrofit Alternatives

6.1. Damage Probabilities

In the LD, the damage probabilities for the SMA bearing retrofit technique indicate a slight reduction in the CD demands under both earthquake scenarios, including the near-field and long-period (Figure 16). On the other hand, a significant reduction in the BDs is observed under both long- and short-period earthquake scenarios when using SMA bearings (Figure 17). Therefore, based on the presented results in the LD of the archetype structure, it is concluded that the SMA retrofit technique is more effective in reducing the BDs than CD demands in this direction.
Figure 18 and Figure 19 also compare the CD and BD damage probabilities for five investigated retrofit techniques in the TD of the archetype bridge. For instance, at 2 × Design (i.e., twice the design intensity, 2D) for long-period earthquakes, it is noticed in Figure 18 that the most effective retrofit technique for reducing CD damage probabilities combines SMA bearings and SLFRSs (i.e., SMA-BRB), while the highest CD damage probabilities are obtained when using the SMA bearings technique. On the other hand, under the effect of short-period events at 5 × Design (i.e., five times the design intensity, 5D), the least CD damage probability is obtained using SLFRSs with the BRB retrofit technique, and the SMA bearings proved to significantly increase CD damage probabilities to reach 74%, as shown in Figure 18. Moreover, at 2 × Design for long-period earthquakes and 5 × Design for short-period events, the SMA bearings combined with SLFRS retrofit techniques (i.e., SMA-SC-BRB) are the most effective techniques for reducing the BD damage probabilities, as depicted in Figure 19.
It is noteworthy that previous studies on existing MSSS bridges retrofitted with different energy dissipation devices indicated a reduction in the probability of damage and improved seismic performance after implementing SMA bearings, which is similar to the present study observations [46]. Moreover, test results of double-column bridge bents retrofitted in a previous study with different BRBs systems indicated that steel braces increased the strength and minimized the residual deformation in RC columns [29]. It is also important to note that the present study combines SMA bearings and different BRB systems to identify the most suitable retrofit technique combination. It is seen in the comparative evaluation results that the SMA-BRB retrofit technique provided the lowest CD demands in the transverse direction, particularly under the far-field scenario. As for BDs, the SMA retrofit technique almost provided the lowest damage probabilities under the far-field scenario. Nevertheless, for the near-field earthquakes, the CD damage probabilities for all retrofit techniques involving SMA bearings were higher than the existing bridge for most performance limit states except for CO, as shown in Figure 14 and Figure 18. This observation is more pronounced for the SMA retrofit approach and was reported in another recent study [46]. As such, the brief comparisons of the present study results with those reported in the literature reflect the consistency in the seismic performance evaluation of different retrofit alternatives with previous relevant results in the literature for comparable RC bridges.

6.2. Selection of Effective Seismic Risk Mitigation Techniques

The adopted range of retrofit approaches revealed different impacts when the archetype bridge is subjected to the long-period and near-field earthquake scenarios in the LD and TD, and Table 2 summarizes the most effective technique for mitigating seismic risk. In the LD of the reference bridge, the retrofit alternative using SMA bearings reduced the seismic demands for both CD and BD damage probabilities. In the TD, the columns’ CD demands controlled the benchmark bridge performance as they reached higher LSEPs at lower ground motion intensities than the BD criterion. Table 2 and the derived fragility functions show that combining SMA bearings and LFRSs (i.e., SMA-BRB retrofit alternative) is the most effective mitigation measure because it results in the highest reduction in CD demands, particularly under long-period records. Although the SMA retrofit technique reduced the BDs even further compared to SMA-BRB, the most controlling bridge component damage criteria is the bridge bents CD, which was not significantly reduced using the SMA bearing alone. Therefore, the SMA-BRB alternative is recommended for retrofitting pre-seismic code MSSS bridges in the TD.

7. Conclusions

This comparative performance evaluation study was directed at selecting the most effective retrofit technique from five alternatives for upgrading MSSS bridges in a seismic region vulnerable to multiple earthquake scenarios. The investigated retrofit alternatives were: (i) shape memory alloy (SMA) energy-dissipating bearings, (ii) conventional buckling restrained braces (BRBs), (iii) self-centered BRBs, (iv) combined SMA bearings and BRBs, and (v) combined SMA bearings and SC-BRBs. Based on the surveyed bridges in the study area, a benchmark structure was selected to represent pre-code, multi-span structures. Previous experimental studies were used to verify the numerical modeling approaches for the adopted retrofit techniques. The benchmark bridge with the five different retrofit alternatives was idealized using a detailed three-dimensional (3D) fiber-based modeling approach, considering different superstructure elements, substructure, abutments, bearings, and expansion gaps. A postprocessor was developed to monitor forces and deformations in different bridge elements representing the superstructure and substructure of the benchmark bridge during inelastic dynamic response simulation.
Based on this comparative performance evaluation study and the developed fragility curves for the benchmark bridge with different mitigation alternatives and seismic scenarios, it is concluded that retrofitting the bridge in the LD using SMA bearings effectively reduced seismic demands. Although different supplementary lateral force-resisting systems (SLFRSs) successfully reduced the curvature ductility (CD) damage probabilities in the transverse direction (TD), they were less effective for reducing bearing displacement (BD) demands. SMA bearings would be a successful retrofit technique if BD demands were the sole critical issue for a substandard bridge. Since two critical issues were identified for the archetype bridge, namely high CD and BD demands, combining two retrofit techniques provides an improved seismic response in the TD than solely using one approach. Therefore, using both SMA bearings and BRBs was the most effective mitigation technique in the TD, especially for critical long-period earthquakes.
It is emphasized that, notwithstanding the investigated number of retrofit alternatives using detailed numerical models and the diverse fragility relations developed for two seismic scenarios, the conclusions were based on an archetype bridge configuration representing typical multi-span simply supported bridge inventory and the adopted retrofit techniques. Therefore, additional studies covering other types of substandard bridges with different structural systems and configurations using additional contemporary retrofit techniques are needed to mitigate seismic losses in the bridge inventory, particularly considering the cost-effectiveness of the various alternatives. This comparative performance evaluation study thus represents a step toward developing a holistic risk management system in the study region using the fragility curves developed and other recent fragility assessment studies.

Author Contributions

Conceptualization, A.M.; methodology, H.G. and A.M.; validation, H.G. and A.M.; software, H.G. and A.M.; investigation, H.G. and A.M.; formal analysis, H.G. and A.M.; resources, H.G. and A.M.; data curation, H.G. and A.M.; visualization, H.G. and A.M.; writing—original draft preparation, A.M. and H.G.; supervision, A.M.; writing—review and editing, A.M.; funding acquisition, A.M; project administration, A.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the UAE University under grant #31N394.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data in this study are available upon request.

Acknowledgments

The support of the UAE University through research grant #31N394 is acknowledged.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Possible seismic mitigation techniques for reinforced concrete bridges.
Figure 1. Possible seismic mitigation techniques for reinforced concrete bridges.
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Figure 2. Selected case study structure: (a) fiber-based numerical model, (b), view showing the bridge superstructure and two-column bents, (c) link element response representing a stopper, and (d) side view showing the dimensions of the superstructure and bridge bent.
Figure 2. Selected case study structure: (a) fiber-based numerical model, (b), view showing the bridge superstructure and two-column bents, (c) link element response representing a stopper, and (d) side view showing the dimensions of the superstructure and bridge bent.
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Figure 3. Comparison between the dynamic response simulation versus experimental results: (a) self-centering steel bracing representing supplementary lateral force-resisting systems (SLFRSs), and (b) shape memory alloy-lead rubber bearing (SMA-LRB) [28,29,32].
Figure 3. Comparison between the dynamic response simulation versus experimental results: (a) self-centering steel bracing representing supplementary lateral force-resisting systems (SLFRSs), and (b) shape memory alloy-lead rubber bearing (SMA-LRB) [28,29,32].
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Figure 4. Curvature ductility demands in the longitudinal direction (LD) for long-period seismic events (left) and short-period earthquake records (right).
Figure 4. Curvature ductility demands in the longitudinal direction (LD) for long-period seismic events (left) and short-period earthquake records (right).
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Figure 5. Bearing displacement demands in the LD for long-period seismic events (left) and short-period earthquake records (right).
Figure 5. Bearing displacement demands in the LD for long-period seismic events (left) and short-period earthquake records (right).
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Figure 6. Fragility functions for CD demands in the bridge substructure under long-period seismic events in the LD (existing vs. retrofitted structure using SMA bearings).
Figure 6. Fragility functions for CD demands in the bridge substructure under long-period seismic events in the LD (existing vs. retrofitted structure using SMA bearings).
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Figure 7. Fragility functions for bridge bearings displacement demands under long-period events in the LD (existing vs. retrofitted structure using SMA bearings).
Figure 7. Fragility functions for bridge bearings displacement demands under long-period events in the LD (existing vs. retrofitted structure using SMA bearings).
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Figure 8. Fragility functions for CD demands in the bridge substructure under short-period events in the LD (existing vs. retrofitted structure using SMA bearings).
Figure 8. Fragility functions for CD demands in the bridge substructure under short-period events in the LD (existing vs. retrofitted structure using SMA bearings).
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Figure 9. Fragility functions for bridge bearings displacement demands under short-period events in the LD (existing vs. retrofitted structure using SMA bearings).
Figure 9. Fragility functions for bridge bearings displacement demands under short-period events in the LD (existing vs. retrofitted structure using SMA bearings).
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Figure 10. Curvature ductility demands in the transverse direction (TD) under the effect of long-period seismic events (top) and short-period earthquake records (bottom) for the retrofitted bridge with five different techniques vs. the as-built structure.
Figure 10. Curvature ductility demands in the transverse direction (TD) under the effect of long-period seismic events (top) and short-period earthquake records (bottom) for the retrofitted bridge with five different techniques vs. the as-built structure.
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Figure 11. Bearing displacement demands in the TD under the effect of long-period seismic events (top) and short-period earthquake records (bottom) for the retrofitted bridge with five different techniques vs. the as-built structure.
Figure 11. Bearing displacement demands in the TD under the effect of long-period seismic events (top) and short-period earthquake records (bottom) for the retrofitted bridge with five different techniques vs. the as-built structure.
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Figure 12. Fragility functions for CD demands in the bridge substructure under long-period seismic events in the TD (bridge retrofitted with five different techniques vs. existing structure).
Figure 12. Fragility functions for CD demands in the bridge substructure under long-period seismic events in the TD (bridge retrofitted with five different techniques vs. existing structure).
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Figure 13. Fragility functions for bearing displacement demands under long-period seismic events in the TD (bridge retrofitted with five different techniques vs. existing structure).
Figure 13. Fragility functions for bearing displacement demands under long-period seismic events in the TD (bridge retrofitted with five different techniques vs. existing structure).
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Figure 14. Fragility functions for CD demands in bridge substructure under short-period events in the TD (bridge retrofitted with five different techniques vs. existing structure).
Figure 14. Fragility functions for CD demands in bridge substructure under short-period events in the TD (bridge retrofitted with five different techniques vs. existing structure).
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Figure 15. Fragility functions for bearing displacement demands under short-period events in the TD (bridge retrofitted with five different techniques vs. existing structure).
Figure 15. Fragility functions for bearing displacement demands under short-period events in the TD (bridge retrofitted with five different techniques vs. existing structure).
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Figure 16. Damage probabilities in the LD for CD under long-period seismic events (left) and short-period earthquake records (right).
Figure 16. Damage probabilities in the LD for CD under long-period seismic events (left) and short-period earthquake records (right).
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Figure 17. Damage probabilities in the LD for BD under long-period seismic events (left) and short-period earthquake records (right).
Figure 17. Damage probabilities in the LD for BD under long-period seismic events (left) and short-period earthquake records (right).
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Figure 18. Damage probabilities in the TD for CD under long-period earthquakes (left) and short-period events (right) at different intensities.
Figure 18. Damage probabilities in the TD for CD under long-period earthquakes (left) and short-period events (right) at different intensities.
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Figure 19. Damage probabilities in the TD for BD under long-period earthquakes (left) and short-period events (right) at different intensities.
Figure 19. Damage probabilities in the TD for BD under long-period earthquakes (left) and short-period events (right) at different intensities.
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Table
Table 1. Long-period (far-field) and short-period (near-source) earthquake records used to assess the archetype bridge.
Table 1. Long-period (far-field) and short-period (near-source) earthquake records used to assess the archetype bridge.
No. Earthquake Name and StationReferenceComponentYearDuration (s)PGA (m/s2)a/v
g/ms−1
1Far-field earthquakesBucharest, Building res. InstituteBUEW1977181.730.60
2Loma Prieta, EmeryvilleEV2601989392.450.57
3Loma Prieta, GGBGGB2701989382.290.61
4Hector Mine, IndioHMI01999600.900.70
5Loma Prieta, OaklandLPO01989402.750.67
6Manjil, TonekabunMATN1321990401.220.76
7Chi-Chi, TAP95TAP95N19991230.960.52
8Near-field earthquakesCoalinga-04, Anticline RidgeCO3952701983153.222.048
9Hollister-04, City HallHOL2711974201.651.480
10Lazio Abr. Y, Cassino, Sant EliaLAEW1984301.121.590
11Livemore-02, Morgan Terr ParkLIV3551980152.242.581
12Montenegro, Petrovac, Hotel OlivaMONY1979280.871.426
13Umbria Ma., Castelnuovo, AssisiUMNE1997451.601.254
14Whittier Narrows-01, Garvey Res., Control BldgWN629601987383.782.432
Table
Table 2. Summary of the most effective retrofit technique under the effect of different seismic scenarios.
Table 2. Summary of the most effective retrofit technique under the effect of different seismic scenarios.
Earthquake Load DirectionLong-Period EarthquakesShort-Period Events
CD *BD **CD *BD **
Longitudinal Direction (LD)SMA BearingsSMA BearingsSMA BearingsSMA Bearings
Transverse Direction (TD)Combined SMA Bearings and LFRSsSMA BearingsLFRSsSMA Bearings
* Curvature ductility of bridge bents (most critical bridge component damage criteria); ** Bearing displacement.
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Ghazal, H.; Mwafy, A. Comparative Performance Evaluation of Retrofit Alternatives for Upgrading Simply Supported Bridges Using 3D Fiber-Based Analysis. Buildings 2023, 13, 1161. https://doi.org/10.3390/buildings13051161

AMA Style

Ghazal H, Mwafy A. Comparative Performance Evaluation of Retrofit Alternatives for Upgrading Simply Supported Bridges Using 3D Fiber-Based Analysis. Buildings. 2023; 13(5):1161. https://doi.org/10.3390/buildings13051161

Chicago/Turabian Style

Ghazal, Homam, and Aman Mwafy. 2023. "Comparative Performance Evaluation of Retrofit Alternatives for Upgrading Simply Supported Bridges Using 3D Fiber-Based Analysis" Buildings 13, no. 5: 1161. https://doi.org/10.3390/buildings13051161

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